Due to weight saving and downsizing of electronic appliances, lithium secondary battery which is nonaqueous electrolyte solution secondary battery having a high energy density and light weight have been used in a wide range of fields. The lithium secondary battery consists mainly of a positive electrode obtained by forming an active material layer containing a positive electrode active material such as a lithium compound whose representative example is lithium cobalt oxide on a current collector; a negative electrode obtained by forming an active material layer containing a negative electrode active material such as a carbon material whose representative example is graphite capable of insertion and extraction of lithium on a current collector; a nonaqueous electrolyte solution generally obtained by dissolving an electrolyte such as a lithium salt including LiPF6 into an aprotic nonaqueous solvent; and a separator formed from a porous film.
The separator used in the lithium secondary battery must meet the requirements of not preventing ionic conduction between the electrodes, retaining the electrolyte solution, having solvent resistance to the electrolyte solution, and the like, and a porous film formed from a thermoplastic resin such as polyethylene and polypropylene is mainly used for the separator.
As a method for producing the porous film, the following methods, for example, have heretofore been known in the art.
(1) An extraction method of achieving a desired porosity by molding a polymer material by adding thereto a plasticizer which can readily be extracted and removed by downstream processing and then removing the plasticizer by the use of an appropriate solvent.
(2) A drawing method of molding a crystalline polymer material and then forming pores by selectively drawing an amorphous region which is structurally weak.
(3) An interface separation method of molding a polymer material by adding thereto a filler and then forming pores by separating an interface between the polymer and the filler through drawing operation.
Since the extraction method (1) requires processing of a large amount of waste fluid, the method has drawbacks in terms of environment and economy as the method for obtaining the thermoplastic resin-based porous film. Also, since it is difficult to obtain a uniform film due to shrinkage of the film caused in the extraction process, the method has a drawback in terms of productivity such as yield. Since the drawing method (2) requires long time thermal processing in order to control a pore diameter distribution by way of control on structures of a crystalline phase and an amorphous phase before drawing, the method has a drawback in terms of the productivity.
As an improved technology of the method (1), JP-A-6-240036 discloses a method of obtaining a polyolefin porous film having a sharp pore diameter distribution (maximum pore diameter/average penetration pore diameter: 1.5 or less) by subjecting to drawing and solvent removal operations at specific temperature a gel sheet obtained by extruding a polyolefin solution containing an ultrahigh molecular weight component and having a large molecular weight distribution followed by quenching.
However, this method is increased in the number of process steps since it is necessary to perform two drawing operations different in temperature for the purposes of forming pores of a uniform size and enlarging the size to a practically suitable one, and, therefore, the method is complicated in process as compared with an ordinary extraction method and has a drawback in terms of productivity. Also, due to the two drawing steps, drawing ununiformity can be caused in each of the steps. Further, since the method substantially is an extraction method, the method requires processing of a large amount of waste fluid as mentioned above to pose a drawback in terms of environment and economy. Also, since it is difficult to obtain a uniform film due to shrinkage of the film during the extraction process, the method has a drawback in terms of productivity such as yield.
In contract, the interface separation method (3) is free from the waste fluid and excellent in terms of environment and economy. Also, since the interface between the polymer material and the filler can be separated easily by the drawing operation, the method makes it possible to obtain a porous film without performing a pretreatment such as a heat treatment and is excellent in terms of productivity. For example, JP-A-2002-201298 discloses a porous film formed from a thermoplastic resin and a filler and having a value of XR of less than 5 when XR is defined by XR=25×TGUR×d2÷Y [wherein Y is a thickness (μm); TGUR is a Gurley value (Gurley air permeability) (second/100 cc); d is an average pore diameter (μm)].    [Patent Literature 1] JP-A-6-240036    [Patent Literature 2] JP-A-2002-201298